FIG. 10A is a diagrammatic illustration of a conventional hydraulic control apparatus for belt type continuously variable transmissions. The delivery pressure of an oil pump is regulated by a PH regulator valve and a PL regulator valve. A high or PH pressure output and a low or PL pressure output from these regulator valves are switched by a four-way valve to supply the PH pressure to a drive pulley and the PL pressure to a driven pulley to change the gear ratio to a high-speed or overdrive (OD) speed gear. The PL pressure is supplied to the drive pulley and the PH pressure is supplied to the driven pulley to change the gear ratio to a low (LOW) speed gear. The PH regulator valve and the PL regulator valve are controlled by a common solenoid valve A, and the four-way valve is controlled by a solenoid valve B.
This conventional hydraulic control apparatus, however, has a drawback that, as shown in FIG. 10B, once the amount of control of the solenoid valve A is determined, the pressure difference between the PH pressure and the PL pressure is also determined, making it impossible to arbitrarily control the speed change time of the belt type continuously variable transmission. Further, with the four-way valve used in the above conventional hydraulic control apparatus the dimension of overlap between a spool groove and a valve body port cannot be controlled easily. Still another problem is that, as shown in FIG. 10C, because of a sharp change in the output hydraulic pressure with respect to the stroke of the spool of the four-way valve, it is difficult to make a fine speed change control.